Primary Visual Cortex Activity along the Apparent-Motion Trace Reflects Illusory Perception
(A) The apparent-motion stimulus consisted of two white squares (size, 1.8°) blinking in alternation (2.3 Hz). The squares were presented at an eccentricity of 9.5° visual angle (distance between squares, 16.5°).
(B) The real-motion stimulus consisted of a white square moving in harmonious oscillation (average speed, 66.5°s−1) on the perceived path of apparent motion. The end points of the movement were adjacent to the position of the apparent-motion squares (maximum eccentricity, 8.0°).
(C) The flicker control stimulus from experiment 2 was composed of two white squares (1.8°) that were blinking simultaneously at 4.6 Hz.
(D–F) For the mapping conditions, high-contrast inverting checkerboards were presented in the upper (D), middle (E), and lower (F) right visual field. Sizes were adjusted according to the cortical magnification factor in V1 to produce activated regions of a similar spatial extent (3.6° for the upper and lower stimuli presented at 9.5° eccentricity, and 1.8° for the middle stimulus presented at 4.7° eccentricity).
(G) For the five conditions of experiment 1 (A, B, D–F), subjects saw a white cross at the middle of the screen, which they had to fixate (G, upper part). In experiment 2, a randomly generated stream of letters and digits was presented at 2 Hz in the middle of the screen (G, lower part). Subjects had to fixate the character stream and either passively view the stream or perform a digit-detection task.
(H) The motion quartet (experiment 3) was composed of four white squares presented at eccentricities similar to the apparent-motion stimulus. Two squares from diagonally opposite corners were presented at the same time. The motion quartet can be seen in vertical (left part) or horizontal (right part) motion without any changes in the physical characteristics of the stimulus. Subjects had to fixate a white cross in the middle of the screen and had to report the perceived direction of movement (vertical or horizontal).